Improve debug message of the layout pass.

Instead of showing multiple lines of debug messages, show only one message
by CompareAtoms::operator(). Here is an example.

Before:
  Sorting _main .text
  Sorting by sectionPos(2,2)
  Sorting by override
  Sorting _main .text
  Sorting by sectionPos(2,2)
  Sorting by override

After:
  Layout: '_main' > '.text' (override (1, 0))
  Layout: '_main' > '.text' (override (1, 0))

Differential Revision: http://llvm-reviews.chandlerc.com/D1964